1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to control of a transmission friction control element during an engine restart event.
2. Description of the Prior Art
A hybrid electric vehicle (HEV) is a vehicle configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio change automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as a battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver's propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver's propulsion request can be provided entirely by the electric motor.
Motor vehicles can be designed to employ certain aspects of hybrid electric vehicle technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, called micro-HEVs, shutting down the engine during conditions where the engine operates at idle speed will be used to reduce fuel consumption in a conventional powertrain that includes an internal combustion engine and a stepped-ratio change automatic transmission, but no electric machine for driving the wheels. The primary condition that is checked by the micro-HEV powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine.
In a micro-HEV application using an internal combustion engine with an enhanced starter motor for engine start/stop and an automatic transmission it is important to provide vehicle propulsion upon an engine restart in a responsive, consistent, and predictable manner. Delays due to transmission engagement and clutch torque capacity application will directly result in wheel torque delays and poor vehicle propulsion response.
Premature clutch torque capacity application can also lead to driveline torque oscillations and potential engine stall while restarting. In addition, poor vehicle performance will be sensed by the driver if the transmission engagement feel is too harsh during or after the engine restart. Temperature and other environmental conditions further amplify these issues.
If the engine is started while the transmission is fully engaged in gear, driveline excitation can occur as a result of engine start transients transmitting to the wheels. Furthermore, in automatic transmission applications with a torque converter, having the transmission geartrain fully engaged with the torque converter unlocked unnecessarily loads the engine while starting. An engine start strategy for which the transmission is fully engaged in gear requires auxiliary hydraulic line pressure during engine stops for electro-hydraulically operated automatic transmissions. This requires an auxiliary pump and results in energy consumption while the engine is off.
A powertrain control system for a micro-HEV powertrain must provide an immediate response to a request for vehicle propulsion when the engine is restarted. A strategy is needed to coordinate clutch filling during an engine start event while minimizing energy consumption in order to provide responsive, smooth, consistent and predictable vehicle propulsion performance.